Although applicable to any gas component, the present invention and its underlying object are explained in greater detail below in relation to the concentration of carbon dioxide in the ambient air.
The detection of carbon dioxide is necessary in many different areas. For example, indoor air in rooms is monitored to determine the ambient air quality or to control ventilation and air-conditioning systems, for example in automotive applications. A limiting value for carbon dioxide, is, for example, 1,000 ppm. Carbon dioxide also occurs in gaseous form in food storage facilities and greenhouses, where it is added to the air, and its concentration must be monitored.
Carbon dioxide sensors are used in general to control concentrations in these applications, and sensors are also desirable in portable warning equipment, the people carrying the warning equipment working in the corresponding areas of application. Warning equipment of this type could also be used in areas where people come into contact with elevated concentrations of CO2, for example of approximately 1%. Applications include, for example, silos and wine cellars, where life-threatening situations can occur if carbon dioxide is not monitored.
Up to now, carbon dioxide sensors have been represented, for example, by electrochemical cells. In this case, the reaction between Nasicon (NaO2) and CO2 should be mentioned. Optical systems based on selective adsorption in the near infrared range by a band in the carbon dioxide spectrum are also used for detection purposes.
In principle, spectroscopic gas sensors of this type include a premolded package, forming a chip package which is manufactured in a molding process by extrusion-coating a specially designed metal carrier strip, known as a lead frame, with plastic or an epoxy resin-based molding compound. According to the related art, the premolded package or lead frames are generally formed in such a way that the connectors needed for electrical connection are provided on at least two side edges or on all four side edges of the package. As a result, the premolded package must be mounted flat on the p.c. board, for example via an adhesive or soldered connection. Once the actual sensor chips have been mounted in the package and are suitably contacted, depending on the application, the package is ordinarily closed by a metal cover having apertures for incident radiation and to prevent stray radiation. The metal cover is usually permanently connected to the edge of the package.
Particularly for optical applications in premolded packages, i.e., for optical chips in a premolded package which is mounted flat on a p.c. board, in which the optical axis of the sensor system is positioned, for example, at a 90° angle relative to the p.c. board, the optical radiation must be deflected over or onto the p.c. board by an additional reflector. Conventionally, an additional reflector of this type is mounted on the p.c. board, and in some circumstances over the premolded package, in an additional assembly step.
However, this conventional approach has proven to have a disadvantage in that it is difficult to orient the reflecting surfaces when mounting the additional reflector on the p.c. board using an adhesive and/or soldering process. Imprecise deflection angles frequently occur, preventing optimum detection of the radiation by the optical chip on the p.c. board.
Furthermore, separate consecutive method steps are required for mounting the metal cover and the additional reflector, which results in a complicated process sequence and an expensive method.